Intaglio printing is a method of printing using depressions enabling thick layers of ink to be deposited on paper, and industrial scale intaglio printing has long been used for stamping, for printing postage stamps, and for printing on banknote paper. In this technique, the print element (a plate or a cylinder) includes engraved depressions made by means of a graving tool and/or by etching, with the depths of the depressions generally being proportional to their widths and lying in the range 0 to 300 microns, said depressions serving to retain ink which is subsequently transferred to the paper.
At present, print elements for printing on banknote paper are made by electroforming, generally using nickel, together with a final layer of chromium plate for surface protection.
The original master is then a plate known as an "artmodel" which is generally made of copper. This plate is engraved by etching and/or by graving tools (and often by both techniques). The engraved artmodel is used for making electroformed nickel print elements which are electroplated with chromium.
The manufacturing process then continues as explained below for plates and cylinders for multi-image printing.
The artmodel is reproduced in as many copies as are required by hot pressure molding in a plastic material (generally polyvinyl chloride). By way of example, it is common practice to use a sheet of 1 millimeter thick polyvinyl chloride and to form molds therein in a press by applying pressure between two plates heated to 100.degree. C. and then allowing them to cool under pressure down to ambient temperature. The average duration of a single cycle is about 20 minutes.
Thereafter, the plastic molds are assembled in a temperature controlled room in order to avoid unwanted variations in dimensions. The molds are assembled to each other by gluing or by high frequency welding, and they are located relative to one another as accurately as possible (to within a few hundredths of a millimeter), generally by means of devices including stepper motors (with three reference axes, x, y, and z, and with rotation about the z-axis).
In a first case, the purpose is to make a printing plate which is then wound around a plate-carrying cylinder.
In this case, the multi-mold plastic assembly is rendered electrically conductive by stannous chloride activation and by spraying with two solutions: a solution of silver nitride and ammonia; and a reducing solution of hydrazine. The assembly is then placed in a bath of nickel sulfamate electrolyte at 40.degree. C. (without chlorine in order to avoid shrinkage) in order to obtain a 1.2 mm thick nickel replica thereof by electroforming for a period of 48 hours. This plate is rectified to a thickness of 0.8 mm and then has a 5 micron thick layer of chromium plating applied thereto in order to harden its surface. The result is a printing plate ready to be wound onto a cylinder. It would naturally be possible to replace the depositing of a very thin layer of silver by a vacuum metallization process, however that would be more complicated to implement and would suffer from the drawbacks due to the inevitable evolution of gas from the plastic material. The above-described technique is thus a kind of electrotype.
In a second case, the purpose is to make a print sleeve which is subsequently fitted on a steel shaft in order to constitute a print cylinder.
In this case, in order to obtain a multi-image nickel sleeve, the above-described assembly of multiple plastic molds, after being made electrically conductive as described above, is inserted in a hollow cylinder constituted by two bronze half-shells. The assembly is placed in a bath of nickel sulfamate electrolyte for three weeks in order to electroform a layer which is 12 mm thick. While still in its bronze shell, the inside of the nickel sleeve is then machined on a lathe in order to obtain constant thickness. It is then removed from the shell and its surface is hardened by 5 microns of chromium plating. The result is a print sleeve ready to be fitted to a shaft.
To complete the picture, it is recalled that there also exists a third case in which the purpose is to make a solid print cylinder, as is used for printing postage stamps. In this case, a milling machine is used in order to obtain a print cylinder bearing the one hundred or the four hundred identical images in the form of depressions This method uses pressure transfer and would not be suitable for printing on banknote paper by virtue of the large areas and depths of the depressions required
In order to obtain a good understanding of the drawbacks presented by nickel print elements (plates or sleeves) obtained by electroforming, it is appropriate to briefly recall the general principles of intaglio printing as conventionally used for printing on banknote paper.
The plate (or sleeve) is mounted on the printing cylinder of a printing press. The cylinder is inked over its entire surface by an inking roll, using a technique which is conventional in printing. Immediately after being inked, a fixed blade or a counter-rotating cylinder fitted with a scraper or with a strip which is itself scraped then removes the ink from the plate (or from the sleeve) leaving ink only in the engraved depressions. Immediately after being scraped, the plate or sleeve is wiped either by means of a strip of paper or else by being sprinkled or brushed with alkali water (water containing about 2% sodium hydroxide and a wetting agent), with said wiping serving to remove all traces of ink from the outside surface of the plate or the sleeve. Thereafter the paper is pressed against the print element by a pressure cylinder which is typically made up of plates of cotton cloth which are clamped under high pressure and filled with a resin (generally polyurethane), and whose outside surface is rectified on a lathe. It is important to observe that the pressure may be as much as one (metric) ton per linear cm. The paper is thus deformed over the depressions from which it picks up the ink. The resulting print is in relief, firstly because the paper is embossed and secondly because of the thickness of the ink unmolded from the depressions.
The first drawback of conventional print elements lies in the rapid wear to which they are subject.
Factors contributing to element wear are ink scraping, paper wiping, and the high levels of pressure used. In spite of the chromium, print elements become worn over periods of time which are unsatisfactory given the size of print runs used for printing paper money. Typically, plates are changed every 750,000 prints and sleeves are dismantled for removal of their chromium plating and for replating with chromium, once every million prints.
Another drawback lies in the need to chromium-plate the printing surfaces in order to obtain the required hardness: chromium plating requires difficult and lengthly operations to be performed and the cylinders need to be repeatedly dismantled.
Another drawback lies in the difficulty of obtaining the degree of polishing required when preparing the ink-receiving surface.
Thus, prior techniques for making intaglio print elements use soft metals that wear relatively quickly and they require difficult and expensive processes to be implemented.
It would naturally be tempting to be able to make print elements that do not wear out, by making them of a hard substance such as steel, for example, and therefore requiring no special maintenance.
However, reproducing engravings repetitively (multi-image printing) matching the original artmodel requires the use of a special mode of machining, since engraving by means of a hand-operated cutting tool is impossible given that the successive reproductions must all be true copies. It has not been possible in the past to engrave on an industrial scale by means of a mechanically operated cutting tool since the dimensional accuracy required is difficult to achieve at a cost/effectiveness ratio that is compatible with realistic industrial operation.
The object of the present invention is to provide a rational and satisfactory method of making a series of intaglio print elements using electro-erosion or spark-erosion machining techniques, thereby making it possible to obtain print elements made of steel.
Electro-erosion machining is a reproduction technique in common use in the motor, aircraft, and nuclear industries, since it makes it possible to obtain very highly accurate machining of materials that are not suitable for machining by any conventional machining techniques (e.g. tungsten carbide). This particular type of machining does not use a cutting tool and it takes place without contact: material is removed by a succession of sparks set up between the piece to be machined and the replica of the shape to be obtained, which replica is electrically conductive and is referred to under these circumstances as an electrode. Machining takes place in machines whose operating cycles are highly automated and which are generally programmable.
A typical electro-erosion machining assembly comprises a generator of intermittent discharges (with the electrical discharges being controlled both in duration and in current) and a frame supporting an electrode (generally made of graphite, brass, or copper) together with a piece to be machined and made of quenched steel. The piece to be machined is fixed in a tank containing a dielectric liquid (generally kerosene) serving to pass the current and to extract the eroded metal, thereby cleaning the gap between the electrode and the piece to be machined. For the work to take place automatically, the electrode must be lowered into the workpiece under servocontrol such that the distance between the electrode and the piece to be machined is maintained at a value corresponding to that thickness of dielectric which can be broken down by the discharges. To this end, the electrode is lowered under servocontrol as a function of the potential difference between the electrode and the piece to be machined.
It is well known in this technique that the rate at which material is removed depends on numerous factors, with the primary factor being the sparking conditions generated by the generator. However, the highest rate at which material can be removed by using powerful sparks at a high recurrence frequency is limited by the desired surface state and by the accuracy with which machining is to be performed.
In practice, in order to obtain a very fine surface state together with high machining accuracy, while nevertheless machining in a short period of time, it is necessary to use several different sparking regimes: roughing; finishing; super-finishing; and polishing.
Both the workpiece and the electrode are subjected to erosion by sparks. However, the difference between the erosion rates on the workpiece and on the electrode depends on the nature of each of them and also on the electrical conditions selected for generating the sparks. By selecting appropriate electrode materials, and by acting on the discharge by varying its duration, its current, and its polarity, it is possible to achieve a high degree of assymetry: e.g. 99% erosion on the workpiece and 0.5% on the electrode. However, under super-finishing and polishing conditions it is difficult to avoid substantial wear on the electrode (about 10%). When excellent machining quality is required, such electrode wear means that electrodes must be replaced progressively as the erosion process moves closer to the polishing stage. For engraving steel, electrodes are generally made of graphite (particularly if high erosion rates are required), but it is also very common to use copper or brass.
The machining performance obtained by electro-erosion is very good. Accuracy within about a micron is easily achieved. The surface state naturally depends a great deal on the sparking conditions: with absolute roughness in microns being about 10 to 20 for roughing, 3 to 5 for finishing, 0.5 for super-finishing, and 0.1 for polishing.
The state of the art is illustrated by U.S. Pat. No. 3,542,993 and by Belgian Patent No. 837 814.
U.S. Pat. No. 3,542,993 describes a technique of making cylindrical punches by means of electro-erosion.
However, the field under consideration is that of cutting out, embossing, stamping, milling, or serrating dies, and not printing. This is important since the degree of accuracy required is quite different: for punches, the desired accuracy is not more than five hundredths of a millimeter at best, whereas for print elements, and a fortiori for intaglio print elements, the desired reproducibility requires an accuracy of five thousandths of a millimeter, i.e. ten times greater.
The mechanical drive means included in the prior art machine for making cylindrical punches would be completely unsuitable for making print elements (see in particular the rack and pinion drive shown in FIG. 15 of the U.S. patent).
It should also be observed that the machine in the U.S. patent seeks, above all, to reduce the time and the cost of making punches, whereas the present invention seeks to make print elements that do not wear out, even if it takes a long time to do so.
Belgian Patent No. 837 814 describes a technique in the field of relief stamping in which a graphic image is engraved on a piece of hard metal by electro-erosion by an electrode including a conducting element on which the image is reproduced by a photogravure technique.
However, here again the field under consideration is very different, and the accuracy requirements are unrelated to those of intaglio printing. For relief stamping it is necessary to obtain an engraving in relief enabling suitable marks to be made, however the required accuracy is at best about half a millimeter.
Thus, the two above-mentioned documents are mentioned as illustrating the technological background of the invention, in particular as illustrating electro-erosion manufacturing techniques in fields which are unrelated to intaglio printing, and in which the degree of accuracy required is small compared with the high degree of fineness required for intaglio print elements.
Further, the summary of prior art techniques for intaglio printing on banknote paper at the beginning of the description of the present application clearly illustrates the complexity of making intaglio print elements, which complexity is naturally not to be found in the field of manufacturing punches for cutting out or for stamping, or in the field of relief stamping.
An essential object of the present invention is to make it possible to duplicate steel print elements, in a manner that is suitable for automation, not only for the purpose of obtaining uniform quality between the print elements made, but also for minimizing, as far as possible, both the time required for obtaining them and the randomness in the process.